EP0584675B1 - Wood pulping with acetic acid and formic acid - Google Patents

Abstract

In a process for producing pulps from lignocelluloses, the production is carried out by heating with aqueous acetic acid under pressure and addition of formic acid, a pulp with a very low residual lignin content being obtained, which can be bleached with ozone and peracetic acid to give high degrees of whiteness, and acetic acid and formic acid being recovered by distillation, so that therefore no effluents arise.

Description

The invention relates to a method for pulp production and bleached obtainable by this process Pulp and chemical pulp. With this procedure you can lignin and hemicelluloses can also be obtained.

Conventional cellulose production processes such as the sulphite and sulphate processes lead to sulfur-containing waste liquors, the combustion of which results in SO ₂ -containing waste gases. The high residual lignin contents of the pulps of 4 to 5% by weight require large amounts of bleaching chemicals, which lead to chlorinated organic compounds in waste water. A further disadvantage of these processes is that plants with a minimum size of 1000 tp cellulose are required because of the chemical recovery by waste liquor combustion.

In US-A-3 553 076 the wood pulping is aqueous Acetic acid described under pressure at 150 to 205 ° C, wherein Pulps with residual lignin contents of 2 to 3% by weight (corresponding Kappa numbers from 12 to 20) can be obtained. After DE-A-34 35 132 wood can be digested even at normal pressure, when the acetic acid catalytic amounts of hydrochloric acid be added (Acetosolv process). Indeed the residual lignin contents of the cellulose decrease not thereby, and chloride ions act in the presence of Strongly corrosive to acetic acid.

Other mineral acids such as sulfuric acid, phosphoric acid, perchloric acid, MgCl ₂ or nitric acid have been investigated as catalysts in wood digestion with acetic acid, which, however, consistently gave pulps with higher residual lignin contents and lead to problems in the recovery of the mineral acids.

Formic acid has also been proposed as a means of digesting wood. In a two-stage process, wood chips were treated with formic or acetic acid in the first stage and hydrogen peroxide was added in the second stage and heated to 70 to 100 ° C. However, the amounts of hydrogen peroxide required for this are too high with regard to economical process management (Poppius et al., Paper and Timber 73 (2) 154-158 (1991).

DE-A-38 30 993 relates to a method for Digestion of lignocellulose-containing material, which as a digestion medium according to a preferred Embodiment at least 90% by weight of a Contains solvent system, this 30 to 45% by weight of formic acid, 5 to 15% by weight of methanol or Includes ethanol and 40 to 60 wt% acetic acid. Of the Digestion takes place at a temperature of 150 to 190 ° C, so an alcohol is a mandatory component this digestion medium. This method enables low Kappa numbers, but requires facilities for Recovery of the alcohol used.

The object of the invention is therefore a method for To provide pulp production, which on the one hand Enables pulps with significantly low residual lignin, on the other hand, however, with regard to its components is simply designed that the recovery of Alcohols can be avoided.

This task is solved by a procedure for Obtaining pulp, lignin and hemicelluloses from Lignocelluloses by heating to 130 to 190 ° C below Pressure in one of primary alcohols and esters of Free formic acid with primary alcohols Digestion medium, the digestion medium consisting of 50 to 95 % By weight of acetic acid, 5 to 40% by weight of formic acid and up to Contains 45% by weight of water.

Wood or annual plants can be used as starting lignocelluloses are used, preferably the Digestion temperature between 180 and 190 ° C. The concentration the acetic acid in the digestion medium is preferably between 50 and 95% by weight, that of formic acid from 5 to 40% by weight and that of water below 50% by weight. The weight ratio of the Lignocellulose for the digestion solution is preferably 1: 1 to 1:12.

According to another embodiment, the method can also for the extraction of lignin and hemicelluloses Lignocelluloses are used. The procedure can be done continuously and discontinuously, wherein in the case of a continuous process, the shredded lignocellulose entered in a pressure cooker by countercurrent of the digestion solution is extracted and the stove on the other side in leaves extra form continuously. You can for example 2 to 20 digestion vessels in a row be scolded.

According to further preferred embodiments, the Pulping and pulp washing process included in the methods of the invention. The lignocelluloses can be used to remove the ingredients pre-extracted with a solvent and the digestion solution can add acetic anhydride and bleach will. According to a further preferred embodiment the lignocelluloses are entered into the Digestion vessel with formic acid, acetic acid, acetic anhydride or impregnated their vapors. The impregnation can also with a solvent or its vapors, that forms an aceotrope with water.

The also with a high degree of purity Lignins and hemicelluloses can be used, for example, to make glue be used.

The process according to the invention has the advantage over the conventional processes for the production of cellulose that it does not use any inorganic pulping chemicals and therefore no SO ₂ -containing waste gases or heavy metal-containing waste water are produced. The pulps can be bleached with ozone in acetic acid and / or hydrogen peroxide, whereby neither chlorine nor waste water containing heavy metals is formed. Formic and acetic acid are recovered by distillation, so that the lignin is obtained by extracting it in countercurrent from the digestion solution and continuously leaving the cooker on the other side in extracted form. For example, 2 to 20 digestion vessels can be connected in series.

According to further preferred embodiments, the Pulping and pulp washing process included in the methods of the invention. The lignocelluloses can be used to remove the ingredients pre-extracted with a solvent and the digestion solution can add acetic anhydride and bleach will. According to a further preferred embodiment the lignocelluloses are entered into the Digestion vessel with formic acid, acetic acid, acetic anhydride or impregnated their vapors. The impregnation can also with a solvent or its vapors, that forms an aceotrope with water,

The also with a high degree of purity Lignins and hemicelluloses can be used, for example, to make glue be used.

The process according to the invention has the advantage over the conventional processes for the production of cellulose that it does not use any inorganic pulping chemicals and therefore no SO ₂ -containing waste gases or heavy metal-containing waste water are produced. The pulps can be bleached with ozone in acetic acid and / or hydrogen peroxide, whereby neither chlorine nor waste water containing heavy metals is formed. Formic and acetic acids are recovered by distillation so that the lignin and hemicelluloses do not need to be burned for chemical recovery. Another advantage is that the digestion temperature of about 10 ° C is lower than in the conventional methods, which significantly reduces energy costs.

The pulp obtained according to the invention has one lower residual lignin content and improved properties. Table 1 shows that the addition of 10% by weight Formic acid under otherwise the same digestion conditions in spruce pulp a reduction in the kappa number of 15.6 to 3.6, corresponding to a lignin content of 2.5 0.5 causes, while the yield drops only slightly.

The same applies to poplar and miscanthus pulp (Tab. 1). The degree of whiteness of the three pulps increases accordingly 8 to 15%. Low kappa numbers and higher whiteness mean less use of expensive bleaching chemicals, the for the economics of the process are important.

Tab. 1 compares the pulp properties after digestion (2h, 180 ° C, poplar and miscanthus 170 ° C) with 85% acetic acid and with 85% acetic acid and 10% formic acid. As can be seen from this table, the strength properties of the pulps obtained with the addition of formic acid also increase significantly. This applies in particular to the tear resistance, which is generally lower in the case of acidic digestion processes than in the case of alkaline ones, for example the sulfate process. Since sulfate pulps are now generally regarded as the standard for paper production, the increase in tear resistance is of particular importance in the process according to the invention. Acetic acid 85% Acetic acid 85% + formic acid 10% Spruce poplar Misc. Spruce poplar Misc. Kappa number 15.6 9.2 13.3 3.6 3.1 3.2 Yield (%) 49.0 50.1 48.5 46.8 50.3 48.2 Whiteness (% ISO) 20.3 20.0 25.9 28.0 34.7 33.8 GVZ (ml / g) 1050.0 1005.0 1022.0 1179.0 849.5 1012.0 DPW 3035.0 2850.0 2910.0 3490.0 2430.0 2870.0 Rip-off (cN) 59.1 31.1 51.5 77.1 42.7 90.4 Bursting surface (m ² ) 62.9 31.4 24.6 70.9 38.0 43.9 Tear length (km) 10.3 7.5 5.2 11.4 7.3 8.1 R-10 (%) 90.1 85.8 88.5 93.6 89.4 91.0 Mannose (%) - 2.3 - 0.5 1.2 0.2 Xylose (%) - 5.4 - 1.5 2.4 3.3 Glucose (%) - 92.3 - 94.2 93.5 96.2

In this table, GVZ mean the intrinsic viscosity Staudinger, DPW the degree of polymerization and R-10 the rest Pulp that is insoluble in 10% Na0H.

The increase in R-10 values also evident from Table 1 in connection with the low xylose and mannose contents means lower hemicellulose levels with the addition of formic acid cellulose obtained and thus its suitability as starting materials (Chemical pulps) for the production of cellulose derivatives. The method according to the invention offers in particular Advantages in the production of cellulose acetate because the pre-swelling of the pulp in acetic acid before acetylation as well as an acetic acid recovery stage are eliminated.

The optimal formic acid concentration depends on the digestion temperature, the digestion time, the type of wood and the water content of the digestion medium. As can be seen from Table 2, at 190 ° C with 20% formic acid, the lignin condensation already prevails after 1 h, which is why in Table 1 a two-hour digestion with 10% formic acid was chosen at 180 and 170%. The acetic acid concentration in Tab. 2 is 85%. Temperature (° C) 190 180 170 Formic acid (%) 5 10th 15 20th 5 10th 15 20th 5 10th 15 20th Kappa number 7.2 5.3 5.6 14.6 14.5 11.2 7.8 7.1 39.9 25.4 14.5 7.1 Whiteness (% ISO) 27.9 29.9 28.7 24.4 22.6 25.8 26.3 27.0 19.8 22.5 24.6 27.6 Yield (%) 46.3 43.6 42.2 42.2 49.2 48.0 46.6 45.3 54.8 50.3 47.0 46.6

Formic acid increases the acidity of the digestion medium and thus the lignin breakdown, while the lignin condensations slow down increase. Compared to mineral acids appears as catalysts the selectivity of formic acid increases during lignin degradation. Formic acid also increases the solubility of Lignins in the exclusion medium.

The chlorine-free bleaching of the process according to the invention cellulose obtained is compared to that of conventional cellulose fundamentally simplified. While with the conventional Today, cellulose bleaching usually has five bleaching stages with oxygen, peroxide, ozone, sodium hydroxide solution and possibly chlorine dioxide are required for the bleaching of the process according to the invention two to three bleaching stages with small amounts of ozone in Acetic acid and / or peracetic acid sufficient.

In the following examples, the percentages refer on the weight.

example 1

Spruce wood chips (20 × 35 × 5-6 mm) with a moisture content of 8% were with 6 times the amount by weight Poured 85% acetic acid containing 10% formic acid and heated in a rotary autoclave to 180 ° C for 2 hours (heating up time 40 min). Thereafter, by evaporating part of the Cooking liquor cooled to below 100 ° C, the pulp on a Nutsche squeezed out and washed with 85% acetic acid. Of the Filter cake was placed in a large beaker with a laboratory mixer whipped under 85% acetic acid and suction filtered again. The pulp obtained was splinter-free and had the properties specified in Table 1.

For comparison, spruce chips with 85% acetic acid, which contained no formic acid under otherwise identical conditions open-minded and worked up. The properties of the pulp obtained under these conditions are also indicated in Tab. 1.

The spruce pulp obtained with the addition of formic acid (Tab. 1) was washed on a nutsche with glacial acetic acid, to a consistency pressed from 35%, fluffed up in a coffee grinder for 30 seconds and then in a round bottom flask on a rotary evaporator gassed with a 3% ozone-oxygen mixture. After that the pulp was first washed with water and on a suction filter then washed with a 0.2% peracetic acid solution in water, pressed to 15% consistency, heated at 80 ° C for 1 h and finally washed with water on the suction filter. The bleached spruce pulp has the specified in Table 3 Characteristics.

In a second approach, spruce pulp was made at 15% Consistency only with peracetic acid, first in acetic acid 0.7% at 80 ° C, 90 min, and then in water with 1.3% at 80 ° C, 120 min, bleached. The results can also be found in Tab. 3.

Example 2

Wood chips (80 × 20 × 5 mm) of a six-year-old poplar (Populus nigra from the clone "Rapp") with a moisture content of 10% was mixed with six times the amount of an 85% acetic acid, which contained 10% formic acid, poured over and two Heated to 170 ° C in a rotary autoclave. The processing, The pulp was defibrated and washed as in Example 1 described for spruce pulp. The pulp properties are shown in Tab. 1.

The pulp was bleached in two stages with peracetic acid, first with 0.7% in 6.6 parts of glacial acetic acid for 90 min 80 ° C, and then with 1.3% peracetic acid in 6.6 parts of water 120 min at 80 ° C. The properties of the bleached pulp are shown in Tab. 3.

Example 3

Stems of Miscanthus chopped to a length of 2.5 cm sinensis "Giganteus" with a moisture content of 18% with ten times the amount of 85% acetic acid, the 10% Formic acid contained, poured over and in a rotary autoclave Heated to 170 ° C for 2 h (heating-up time 40 min). Refurbishment, defibration and the pulp was washed as in Example 1 described for spruce pulp. The pulp was splinter-free. Its properties result from Tab. 1 and are those of under the same conditions, but excluding acetic acid obtained pulps compared.

The pulp was bleached in two stages with peracetic acid, as described for poplar cellulose under example 2. The properties of the bleached pulp can be found in Tab. 3.

Example 4

Spruce woodchips of the type specified in Example 1 were poured over with six times the amount of 85% acetic acid, which contained 5, 10, 15 or 20% formic acid in four batches, and were each heated to 190 ° C. in a rotary autoclave for 1 hour. The pulp obtained was worked up, defibred and washed in the same manner as in Example 1. The pulps were then free from splinters. Their residual lignin contents, degrees of whiteness and yields are shown in Table 2. Properties of the pulp bleached with ozone (Z) and peracetic acid (Pa) at 20 SR freeness cellulose Bleach Amount (%) Whiteness (% ISO) Tearing length (km) Bursting area (m ² ) Rip-off (cN) Spruce Z / Pa 0.6 / 1.3 84.3 9,315 52.5 76.7 Father 0.7 / 1.3 72.1 9.113 50.3 79.1 poplar Father 0.7 / 1.3 83.4 6.68 28.8 46.0 Miscanthus Father 0.7 / 1.3 83.0 6,933 35.6 89.6

Example 5 A. Information

Cooker: 2.5 × 10 m = 49 m ³

Digestion solution: acetic acid / water / formic acid (75:15:10)

Temperature: 160-180 ° C

Time: 1 - 2 h

Fleet ratio: 1: 5

A cooker is sufficient for the test phase (batch process, 25 t pulp / d) while for production 6 to 12 cookers can be connected in series (semi-continuous Process, maximum 300 t of pulp / d). Only by connecting several cookers in series an extraction of the wood chips according to the countercurrent principle possible with optimal use of the digestion solution. The wood chips are heated by pumping the digestion solution heated externally in heat exchangers.

B. bleaching with hydrogen peroxide

The first bleaching stage is carried out with 1 to 2% hydrogen peroxide in the cooker after the digestion has ended and the extract has been displaced by fresh digestion solution, 1 to 2 hours at 70 to 90 ° C. The H ₂ O ₂ is evenly distributed by pumping around the bleaching solution, the composition of which, with the exception of the H ₂ O ₂ , does not differ from the digestion solution. The active agent is peracetic acid, the formation of which is catalyzed by the formic acid present.

C. Sorting

The sorting consists of a post-defibrillation (separation), Rough sorting and cleaning. For the first Both steps will be equipped with a mixing arm Slot sorter, slot width about 0.4 mm, like a Pipe centrifuge and a hydrocyclone for cleaning suggested. The diffuser (thickener) must be very effective to be of a consistency of about 1%, which for the Sorting is required to come to at least 8% from which a consistency of about in a screw press 40% for ozone bleaching must be achieved.

In the sorting process, the cellulose washing takes place at the same time. A separate wash, as with the conventional processes, is not necessary as no inorganic Digestion chemicals are to be washed out and the cooker leaving pulp hardly contains any lignin. The effectiveness the sorting can be done in series Improve pipe centrifugation or cyclones.

D. Ozone bleaching

Ozone bleaching takes place in a rotary drum at 20 to 50 ° C and a consistency of about 40%, the residence time of the pulp should be at least 10 minutes, ozone amount about 0.5% calculated on pulp. Way of the good Solubility of the ozone in acetic acid is a puffing up of the pulp is not required.

Because of the risk of explosion of acetic acid vapors Oxygen-ozone mixtures is explosion-proof Execution of the rotating drum required. The exhaust gases should be in a loop or a closed system being held. Ozone discharged with the pulp decomposes within a certain time. A checkpoint appears in the distillation column absolutely necessary especially with high ozone doses (0.5%). Perhaps would have the acetic acid / butyl acetate mixture to be distilled degassed in vacuum or the excess Chemical ozone is removed (test with KJ solution).

E. Solvent exchange

After ozone bleaching, the pulp still contains about 60% Digestion solution in an extraction tower (2.0 × 10 m) is displaced with butyl acetate. Because the swelling of the pulp in acetic acid is stronger than in butyl acetate, should be in the tower there are no constipation problems.

The amount of butylacetate together with the digestion solution leaves the tower at the head, should be about 60% of the dry weight of the pulp if the Wood moisture of the wood chips is 10% because then the subsequent distillation 20% water, based on Pulp weight as the aceotrope with butyl acetate Distillation column leaves in the upper part. Under these The butyl acetate would condition the mixed solution leave completely as an aceotrope while still 2.5% water remain in the digestion solution, which is undistilled flows back into the storage tank. Small amounts of Extract substances, furfural etc. remain in the digestion solution and do not disturb the information. A separation of the Formic acid is from acetic acid by distillation not mandatory. Changes in the composition of the digestion solution (see under A.) are by adding the in Compensate deficit existing component.

F. Exchange the butyl acetate for water

The butyl acetate is exchanged for water with steam in a desolventizer. The consistency of the pulp is brought to about 40% by a screw press upstream of the desolventizer. Since the enthalpy of vaporization of the butyl acetate is only about 1/5 that of water, the pulp leaves the desolvent coating with 12% moisture and is then pressed in a press into sheets of 1 m ² .

G. Evaporation of the waste liquor

The waste liquor leaving the stove contains 17% dissolved Lignin and hemicelluloses. Your focus on one 50% thick liquor takes place in a six-stage tube evaporator with pressure drop using the heat of condensation the vaporized digestion solution.

H. distillation

The distillation column only serves to separate the with the chips of water entered from the digestion solution as an aceotrope with butyl acetate. After distillation the digestion solution flows undistilled into the storage tank. The capacity of the column is directed after the wood chips moisture. If this is 10%, so are 200 kg of water per ton of pulp (together with approx 600 kg of butyl acetate).

Since the digestion solution contains 15% water, 1.333 m ^{3 of} digestion solution + 0.6 m ^{3 of} butyl acetate would be required per ton of pulp. With a wood chips moisture of 20%, the amount doubles. It should therefore be considered whether the wood chips can be pre-dried, which would also be advantageous for the storage capacity of the wood chips. Drying the wood chips has no influence on the wood digestion using the Formacell process.

I. Spray drying the thick liquor

Laboratory and pilot plant experiments have a powder with them about 1% acetic acid.

K. General comments

For all parts of the plant with the hot digestion solution come into contact, steels must be used that corrosion-resistant to acetic acid / formic acid / water mixtures are. This is especially true for the cooker that Distillation column and the spray dryer.

Pulp grades

Tab. 4 compares the properties of unbleached spruce pulps obtained by the process according to the invention (Formacell process) with those of sulfate and acetosolv pulps. Formacell sulfate Acetosolv Kappa number 3.6 30.6 15.6 Whiteness (% ISO) 28.0 24.8 20.3 GVZ (ml / g) 1179.5 902.2 1059.0 DP 3490 2470 3035 R-10 (%) 93.6 88.3 90.1 Yield (%) 46.8 47.4 49.0

The very low kappa number of the Formacell pulp is striking compared with only slightly reduced yield the conventional sulfate pulp, which is essential requires less bleaching chemicals. Because of Formacell pulps are suitable for their high R10 value also for the production of cellulose derivatives.

Tab. 4 also shows the improvements to the Formacell process compared to the previous Acetosolv process, which especially in a significant improvement in Delignification and increase in whiteness.

Fig. 1 shows a comparison of the strengths (tear length and tear resistance) of Formacell and sulfate spruce pulp depending on the degree of grinding.

During the tear length of Formacell spruce pulp all grades above the values of the sulfate pulp is the tear resistance of the Formacell pulp lower. Overall, the strength potential of the Formacell pulp with that of sulfate pulp in about comparable.

Even more favorable values are obtained with Miscanthus pulps, as can be seen from Table 5. Here not only lower kappa numbers and higher degrees of whiteness are obtained, but also significantly higher tear resistance than with the conventional soda process. In recent investigations, even higher tear resistance, which comes close to that of spruce sulfate pulps, could be obtained. Since the Formacell process, in contrast to the soda process, does not produce lye containing sodium silicate, it is particularly suitable for the digestion of annual plants. Properties of Miscanthus pulps obtained by three different processes Formacell soda Acetosolv Kappa number 3.2 27.2 13.2 Whiteness (% ISO) 33.8 26.6 25.9 GVZ (ml / g) 1012 1010 1022.5 DP 2870 2870 2910 R-10 91.0 - 88.5 Yield (%) 48.2 54.6 48.5 Penetr. (cN) 90.4 63.2 51.5 Bursting Fl. (m ² ) 43.9 41.2 24.6 Tearing length (km) 8.1 7.08 5.2

Claims (14)

1. A process for obtaining celluloses, lignin and hemicelluloses by means of heating to 130 to 190°C under pressure in a pulping medium which is free of primary alcohols and of formic acid esters with primary alcohols, characterized in that, the pulping medium contains 50 to 95% by weight of acetic acid, 5 to 40% by weight of formic acid and up to 45% by weight of water.
2. A process in accordance with claim 1, characterized in that, the lignocellulose is wood or an annual plant.
3. A process in accordance with one of the preceding claims, characterized in that, the moisture content of the lignocellulose can be reduced by means of a pretreatment of increased temperature or by means of solvent vapors.
4. A process in accordance with one of the preceding claims, characterized in that the weight ratio of the lignocellulose to the pulping solution amounts to 1:1 to 1:12.
5. A process in accordance with one of the preceding claims, characterized in that, the crushed lignocellulose is continuously fed into a pressure cooker in which it is extracted, in the counter-current, from the pulping solution, and continuously leaves the cooker at the other side, in an extracted form.
6. A process in accordance with claim 5, characterized in that, a continuous extraction is obtained through the fact that 2 to 20 pulping vessels are connected in series one after the other, and the pulping solution extracts the lignocellulose in the counter-current.
7. A process in accordance with one of the preceding claims, characterized in that, the shredding of the cellulose is included.
8. A process in accordance with one of the preceding claims, characterized in that, the washing process of the cellulose is included in the continuous extraction.
9. A process in accordance with one of the preceding claims, characterized in that, the lignocellulose is pre-extracted with a solvent for the removal of its contents.
10. A process in accordance with one of the preceding claims, characterized in that, acetic anhydride is added to the pulping solution.
11. A process in accordance with one of the preceding claims, characterized in that a bleaching agent is added to the pulping solution.
12. A process in accordance with one of the preceding claims, characterized in that, the lignocellulose is, before being introduced into the pulping vessel, impregnated with formic acid, acetic acid, acetic acid anhydride, or the vapors of the same.
13. A process in accordance with one of the preceding claims, characterized in that, the lignocellulose is impregnated with a solvent or with the vapors of the same, which forms, with the water, an azeotrope.
14. A process in accordance with one of the preceding claims, characterized in that, it includes the bleaching of the cellulose.

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